ROAS bidding

I am currently doing ROAS bidding on my display and RMKT campaigns. The big change in turning on ROAS is my CPCs increased significantly but not with total value. As I realise it might be a too high ROAS value that causes the big lift in CPC, I tried to lower the expected ROAS value. But the CPCs stay the same and the total value is not increasing either. Can anyone offer me suggestions in optimize ROAS bidding campaigns?